The mammalian ventrobasal thalamus (VB) is the location of the final processing of somatosensory information before it is relayed to the cerebral cortex. Most of our knowledge of the somatosensory thalamus relates to thalamocortical relay (TCR) neurons while relatively little is known about the local circuit neurons (LCNs) which comprise approximately 25% of the VB neuronal population. As the LCNs are known to be GABAergic and constitute a major component of the inhibitory processes in the thalamus these neurons play a central role in thalamic function. Knowledge of the anatomical and physiological organization of the VB is necessary to understand the bases of somatic sensation and particularly the differences between the relay of noxious (pain producing) and non- noxious stimuli. The research proposed here is designed to study the morphology of local circuit neurons (LCNs) in the cat VB complex. The study will use both anatomical and physiological methods to examine the synaptic input to LCNs. In the anatomical studies, LCNs will be identified immunocytochemically by the presence of GABA. The number and distribution of synaptic terminals derived from the dorsal column nuclei (DCN), spinothalamic tract (STM, cerebral cortex and thalamic reticular nucleus (TRN) will be determined using axonal transport (PHA-L) or degeneration methods. A second series of experiments involve the physiological characterization (i.e. receptive field properties) and subsequent intracellular HRP filling of LCNs. The morphology and axonal projections of the filled LCNs will be studied at the LM level. This study will provide information on the synaptic connectivity of LCNs similar to that presently available for TCR neurons. It will be determined in what way the synaptic organization and response properties of LCNs may affect the transfer of somatosensory information through the thalamus. Differences between the organization of pathways associated with noxious and non-noxious sensation will provide insight into the role of the thalamus in pain mechanisms.